Even in dry times, Waller Creek in Austin, Texas, is never without water. That is because a significant portion of the water that flows through it — anywhere from 25-50% — can be traced to wastewater from leaky city pipes.

But there is a silver lining to the less-than-perfect plumbing, according to new research. The water flowing through the creek, located on the University of Texas Austin campus, sustains trees growing along it, allowing them to thrive during drought conditions that take a toll on others growing along streams in more rural areas.

The research highlights how urbanisation can have unintended positive effects among more well-known negative effects, such as pollution and higher levels of illness-causing bacteria.

Professor Jay Banner from the Jackson School of Geosciences at the university said, “Those negative effects are not cancelled out. One has to weigh the unintended positive consequences with the expected and long-shown negative consequences.”

Researchers compared the growth record of bald cypress trees along Waller Creek and Onion Creek, a rural stream about 12 miles away, to the drought record of the region. A tree’s growth is recorded in tree rings in its wood, with thicker rings reflecting wetter times and strong growth and thinner rings reflecting dryer times and little growth.

Banner and his students extracted this record from living trees by taking cores - narrow rods of wood pulled from tree trunks - and sending them to the Laboratory of Tree-Ring Research at the University of Arizona for analysis.

Analysis of growth rings in the core revealed that the Waller Creek trees have always had a dependable water supply, suggesting it has long been a conduit for water leaking from surrounding pipes.

“The trees needs are being met. They’re not being stressed by the climate,” said co-author Bryan Black, an associate professor at the University of Arizona.

“It fits right in line with the hypothesis that they’re receiving supplemental water though this urban infrastructure leakage.”

In addition to sustaining the trees through drought, the constant flow of water also appeared to make the Waller Creek trees take on an individualised growth pattern, with different trees having different growth rates throughout the year. The researchers think that with their water needs met, other factors that affect tree growth - such as genetics, competition from other trees, nutrients, or the presence of pests - took on a bigger influence.

Although this would be an unusual result in a rural setting, researchers said they were not surprised to see the Waller Creek trees doing their own thing.

“That’s kind of what we expect in an urban environment,” Banner said. “We would expect the natural system to be, well, for lack of a better term, messed up.”

Ongoing research is investigating how the chemistry of the tree rings may reflect the timing and amount of this municipal water input, as well as distinguishing between the municipal water sources of wastewater, tap water, and irrigation.

Leaky infrastructure is commonplace across cities worldwide. That means there are probably pockets of trees being sustained by wastewater like those along Waller Creek, the researchers said. Understanding the extent of infrastructure leakage and its ecological influence can help policymakers understand the broader effects of urbanisation — the good and the bad — and plan accordingly.

The findings were published in the Nature Partner Journal Urban Sustainability.